Coatings and materials can become soiled from debris (particles, insects, oils, etc.) impacting the surface as well as ice forming on the surface. The debris and ice affects airflow over the surface.
In aviation, icing conditions are those atmospheric conditions that can lead to the formation of water ice on the surfaces of an aircraft, or within the engine as carburetor icing. Inlet icing is another engine-related danger, often occurring in jet aircraft. Icing conditions exist when the air contains droplets of supercooled liquid water. The wing will ordinarily stall at a lower angle of attack, and thus a higher airspeed, when contaminated with ice.
If ice is present on an aircraft prior to takeoff, the ice must be removed from critical surfaces. Removal can take many forms, including mechanical means, deicing fluids, hot water, or infrared heating. These techniques may remove existing contamination, but provide no practical protection in airborne icing conditions. Deicing fluids may resist the effects of snow and rain for some time but are intended to shear off the aircraft during takeoff and therefore provide no inflight protection.
To protect an aircraft against icing in-flight, various forms of anti-icing or deicing are used. Some aircraft are equipped with pneumatic deicing boots that disperse ice build-up on the surface. A weeping wing system may be used, with many small holes that release anti-icing fluid on demand to prevent the buildup of ice. Electrical heating may be used to protect aircraft and components (including propellers) against icing. Modern commercial aircraft often employ a hollow tube located behind the leading edge of the wing, through which hot engine bleed air is directed to melt and release ice.
Passive, durable anti-icing coatings have been identified as a need in the aerospace field for many decades. However, previous solutions lacked a required level of performance in ice adhesion reduction, adequate long-term durability, or both of these. Some of the most-effective coatings for reducing ice adhesion are dependent on sacrificial oils or greases that have limited useful lifetimes and require regular reapplication. Currently, durable coatings for exposed areas on fixed wing and rotorcraft (such as the leading edge of the wing or rotorblade) include thermoplastic elastomers bonded to the vehicle surface using a film adhesive or an activated adhesive backing incorporated into the coating itself. However, the prior compositions do not provide any benefit in lowering ice adhesion.
There remains a desire for coatings on aircraft exteriors (and other aerospace-relevant surfaces) in order to passively suppress the growth of ice near strategic points on the vehicle—such as the rotorblade edge, wing leading edge, or engine inlet. There also exists a need for high-performance coating materials fabricated in a way that preserves coating function during actual use of aerospace structures.
AMIL is the Anti-icing Materials International Laboratory located at the Université du Québec à Chicoutimi in Chicoutimi, Quebec, Canada. The icephobic character of a coating can be evaluated by measuring the ice adhesion reduction effect of a candidate coating compared to an uncoated surface. AMIL can evaluate icephobic coatings in many different atmospheric conditions (wind and temperature) with glaze or rime accreted ice obtained with a simulation of freezing precipitation.
A single “Centrifuge Adhesion Test” by AMIL consists of the ice adhesion measurement of three small aluminum beams covered with the candidate product, compared with three bare beams. The extremity of the six sample beams are iced simultaneously with freezing precipitation on about 5 cm2 surface to a thickness of about 7 mm. Each sample beam is rotated and balanced in the centrifuge apparatus. The rotation speed increases with a constant acceleration rate until the centrifugal force resulting from rotation reaches the adhesion stress of ice, detaching the ice. This detachment is picked up by a piezoelectric cell (sensitive to vibrations) which relays signals in real time to a computer. Finally, the adhesion stress is calculated using detachment speed, the mass of ice, and the beam length.
The Adhesion Reduction Factor, ARF is calculated using the average stress measured on the three coated beams compared to the average stress measured on the three bare (control) beams. In particular, from the centrifugal force the stress is determined as F=mr ω2 where F=centrifugal force [N], m=mass of ice [kg], r=radius of the beam [m], and ω=speed of rotation [rad/s]. The Adhesion Reduction Factor (AMIL ARF) is then calculated using the average stress measured on the three coated beams compared to the average stress measured on the three bare beams: ARF=τbare/τcoated where τbare=average stress measured on three simultaneously iced bare beams [Pa] and τcoated=average stress measured on three simultaneously iced beams with candidate icephobic coating [Pa]. The web site www.uqac.ca/amil/en/icephobiccoatings/centrifuge, as retrieved on the filing date hereof, is hereby incorporated by reference herein.
An ARF value of 1 means there is no icephobic effect. An ARF value greater than 1 means there is an ice-adhesion reduction (icephobic effect); the higher the value, the more icephobic (low ice adhesion) the coating.
Low-ice-adhesion coatings are certainly not limited to aerospace-relevant surfaces. Other potential applications would include wind turbine blades, automobiles, trucks, trains, ocean-going vessels, electrical power transmission lines, buildings, windows, antennas, filters, instruments, sensors, cameras, satellites, weapon systems, and chemical plant infrastructure (e.g., distillation columns and heat exchangers).
In view of the shortcomings in the art, improved low-ice-adhesion coating materials and material systems, and compositions suitable for these systems, are needed.